Osteoporosis is an aging related condition characterized by progressive loss of bone mass and increasing bone fragility that affects many Veteran men and women. Given the tremendous morbidity and excess mortality associated with osteoporosis-related fracture, it is vital to the VA healthcare mission to explore novel strategies to reduce fracture risk. The only current FDA-approved anabolic therapy for severe osteoporosis is teriparatide [hPTH(1-34)], a conventional agonist of the type 1 parathyroid hormone receptor (PTH1R). While effective at stimulating osteoblastic bone formation, its clinical utility is limited by unfavorable pharmacokinetics necessitating daily injection, its propensity to promote hypercalcemia, a lack of efficacy in states where endogenous PTH is elevated, and the rapid loss of accrued bone mass upon discontinuation of treatment. The ?ideal? anabolic agent for osteoporosis would circumvent these limitations, i.e. it would be capable of stimulating osteoblastic bone formation with prolonged exposure, and would ?uncouple? increased bone formation rates from accelerated bone resorption and renal calcium retention. One approach that has shown early promise is the use of ?biased? PTH1R agonists that stimulate receptor signaling through non-G protein effectors, e.g. arrestins, while antagonizing receptor coupling to heterotrimeric G proteins. ?Biased? ligands are novel pharmacological entities that possess the unique ability to engender ?mixed? agonist-antagonist effects in vivo that cannot be attained through conventional agonism or antagonism. In preliminary work, we have demonstrated that a modified bovine PTH derivative, [D-Trp12,Tyr34]-bPTH(7-34), can bias PTH1R in a manner that dissociates heterotrimeric G protein-dependent signaling from non-canonical arrestin-dependent signaling. In vivo, [D-Trp12,Tyr34]-bPTH(7-34) promotes osteoblast differentiation and survival leading to increased bone formation in eugonadal male and female mice, while antagonizing the G protein-mediated signals that lead to osteoclast activation and calcium retention. Functional genomic analysis of bone from mice treated with conventional and biased PTH1R agonists indicates that despite targeting the same receptor, [D-Trp12,Tyr34]- bPTH(7-34) and hPTH(1-34) have distinct mechanisms of action. The overall objective of this proposal is to determine whether an arrestin pathway-selective ?biased? PTH1R ligand affords specific advantages over teriparatide in terms of reduced side effects and retained efficacy in settings where a conventional PTH1R agonist is ineffective or contraindicated. Our central hypothesis is that a ?biased? ligand that uncouples arrestin-dependent PTH1R signals that increase osteoblast number/activity from G protein-mediated signals that stimulate osteoclastic bone resorption, will likewise uncouple the beneficial effects of PTH1R activation on bone formation from its adverse effects on bone resorption and renal calcium retention in vivo. The project consists of three specific aims, each of which compares conventional and ?biased? PTH1R agonists in a different pre-clinical rodent model of metabolic bone disease. Aim 1 tests whether an arrestin pathway- selective PTH1R agonist will promote net accrual of bone mass when administered by continuous infusion, a dosing regimen under which conventional PTH1R agonists may cause net bone loss through overstimulation of osteoclast activity. Aim 2 tests whether an arrestin pathway-selective PTH1R agonist, like a conventional agonist, will promote accrual of bone mass under conditions of sex steroid withdrawal, but do so without accelerating bone turnover or causing hypercalcemia/hypercalcuria. Aim 3 tests whether an arrestin pathway- selective PTH1R agonist will promote accrual of bone mass in the setting of secondary hyperparathyroidism induced by renal failure, a setting in which conventional PTH1R agonists are contraindicated due to already high circulating levels of endogenous PTH. Completion of this project will provide key pre-clinical tests of arrestin pathway-selective biased agonism as a novel anabolic approach to osteoporosis treatment, potentially leading to the development of novel therapeutics that will benefit many Veteran men and women.